•    Studies have shown that a positive ultrasound of the lower limb has a high specificity for the diagnosis of pulmonary embolism in the right clinical setting, ruling in of the diagnosis of  PE without further testing in these patients.  
•  The data have demonstrated the high diagnostic utility of a limited, two-site compression ultrasound exam
•  Only the branch points of the common femoral and popliteal veins need to be examined, as the majority of symptomatic DVTs include these sites.
•  It is important to note that a negative US does not rule out a DVT (or PE), due to low sensitivity and low NPV (as there are many areas inaccessible during the bedside examination)

Compression Technique:
-          The patient should be in a supine position with the leg externally rotated and knee flexed.
-          Using a linear probe, in the transverse orientation, apply firm compression to achieve complete collapse of the vein.
-          The lumen of the vein must disappear completely in order to rule out the presence of a clot.
-          You may be able to visualize the clot as echogenicity within the vessel; but often the only evidence of DVT is the inability to fully compress the vein.

 
Two Points:

Common Femoral Vein: In the inguinal area, locate where the great saphenous vein joins the common femoral vein. The common femoral artery will be located laterally.

Popliteal Vein: Place the probe behind the patients knee in the popliteal fossa. The popliteal vein lies superficial to the artery.

Below is an example of a positive DVT ultrasound, in which there is incomplete collapse of the common femoral vein, indicating the presence of a thrombus.

Summary: Ultrasound can be a useful diagnostic tool in the rapid evaluation of patients presenting with signs and symptoms suggestive of a PE as it can decrease time to diagnosis and treatment when a positive result is found on examination.

Special thanks to Alfred Cheng, MD for the ultrasound images.

References:

ACEP Clinical & Practice Management. Focus On: Emergency Ultrasound for Deep Vein Thrombosis. ACEP News. March 2009.

https://www.acep.org/clinical---practice-management/focus-on--emergency-ultrasound-for-deep-vein-thrombosis/

Soni. Point-of-Care Ultrasound. Elsevier, 2015. Print. Pages 211-215.

Le Gal, G. et. al. A positive compression ultrasonography of the lower limb veins is highly predictive of pulmonary embolism on computed tomography in suspected patients. Throm Haemost. 2006 Jun;95(6):963-6.

Roy PM, et. al. Systematic review and meta-analysis of strategies for the diagnosis of suspected pulmonary embolism. BMJ. 2005 Jul 30;331(7511):259.

Konstantinides, SV. et al. 2014 ESC guidelines on the diagnosis and management of acute pulmonary embolism. Eur Heart J. 2014 Nov 14;35(43):3033-69.